Here's a tough question, if you hook up a computer power supply to power some leds only, will the power supply be drawing the 250W-1000W it's rated for or will it be more efficient? Does it vary from power supply to power supply? Using a PC power supply to light 16 led's could potentially be a huge waste of power. take a look at some of the led drivers they have at ledsupply.com, even if you have to drop $40 on a 12W power supply, the energy saving costs could far outweigh the price, if in fact the power supply is pulling 100 Watts or more.
power supplies are only moderately efficent to begin with. it's alot better than they used to be. You aren't going to be using all 1000 watts or whatever in your power supply. it's just an outline of what the supply is capable of. If you take a look at your specs for your supply, you will find the bulk of the wattage available is reserved for the 3.3 volt and 5 volt lines. the 12 volt line is somewhere in the neigborhood of 100 or 200 watts (about 20 amps)
your 1000 watt powersupply is made up of basically 200+300+500 watt supplies (12/5/3.3 volt supplies) added together (total 1000 watts)
only using a portion of the 12 volt out of the WHOLE supply is only using a fraction of it's power.
where things get tricky is that the efficiency is low (under 80%) under low load...gets better as the load goes up (90% or better)... then goes back down again as the load approaches maximum.
the actual values where this can happen vary WILDLY even between supplies of the same run.
if efficency is important, use a amp meter (or the amp function of your volt meter if it has one) and measure the ACTUAL load at full operation. Let's say it's 2147mA. I would then allow a little overhead and try to source a 2.5 or 3 amp power supply...
the load is close to a full 2500mA (or 3000mA) so it's efficent, but not so full your taxing the supply or running into the overload area. I would think a 4 amp supply (4000mA) will be in the low load area.